Surface acoustic wave filters are successfully used in wireless communication systems. Their filtering function is provided by resonator-type structures, built on piezoelectric substrates with high electromechanical coupling factor. Such devices commonly utilize low-attenuated leaky surface acoustic waves (LSAW) characterized by high electromechanical coupling factor. By way of example, such waves are known to exist in two piezoelectric crystals belonging to the same symmetry class 3m, lithium tantalite (LiTaO3 (LT)) and lithium niobate (LiNbO3 (LN)).
Conventional SAW devices must be hermetically packaged and require air cavity above the surface of the die on which the SAW propagates. In 1924 the Stoneley wave was discovered. The Stoneley wave is tightly bound to the interface between the two solid half-spaces, FIG. 1. The field distribution is entirely composed of two partial waves (longitudinal wave and shear vertical wave) decaying away from the surface in each medium, FIG. 2. A simple illustration of a Stoneley wave propagating in the direction of the x-axis and decaying in along the z-axis is illustrated in FIG. 3.
Reference is made to R. Stoneley, “Elastic waves at the surface of separation of two solids,” Roy. Soc. Proc. London, Ser. A, 106, 1924, pp. 416-428, the disclosure of which is herein incorporated by reference.
The polarization of the Stoneley wave, FIG. 2, is dominated by the Shear Vertical (SV) displacement, with a small longitudinal displacement. The Stoneley wave has no Shear Horizontal displacement (SH). The Stoneley wave may be described as a SV type, or more appropriately as a quasi-SV type.
Auld describes the conditions for the existence of Stoneley waves at the interface between two isotropic and between two cubic materials.
Reference is made to B. Auld, “Acoustic Fields and Waves in Solids,” Vol. II, Robert E. Krieger Publishing Company, 1990, the disclosure of which is herein incorporated by reference.
Due the underlying physics the Stoneley wave only exists under special conditions and its existence and is very sensitive to the velocities of the Rayleigh and shear waves of the denser medium. For example, in isotropic materials the Stoneley (or Boundary) wave's velocity (VB) may only exist between the velocity of Rayleigh wave (VR) and velocity of the shear wave (VS) of the denser of the two materials (the upper and lower half-spaces).
In 1978 Yamanouchi et al, considered piezoelectric boundary acoustic waves (PBAWs) propagating at the interface between 127° YX LT and SiO2. The polarization of the PBAW considered by Yamanouchi was proportionately longitudinal. The second largest displacement is the Shear Vertical (SV) type, and the SH is the smallest. FIG. 5 illustrates the polarization of this wave. Yamanouchi's wave is a quasi-longitudinal type.
Reference is made to K. Yamanouchi, K. Iwahashi and K. Shibayama, “Piezoelectric acoustic boundary waves propagating along the interface between SiO2 and LiTaO3,” IEEE Trans. Sonics and Ultrasonics, vol. su-25, No. 6, 1978, pp. 384-389, the disclosure of which is herein incorporated by reference.
The wave described by Yamanouchi remains sensitive to the upper and lower half-space properties. Hashimoto et al have published the sensitive conditions for which a PBAW may exist.
Reference is made to K. Hashimoto, Y. Wang, T. Omori, M. Yamaguchi, M. Kadota, H. Dando, and T. Shibahara, “Piezoelectric Boundary Acoustic Waves: Their Underlying Physics and Applications,” IEEE Ultrasonics Symposium Proceedings, 2008, the disclosure of which is herein incorporated by reference.
In 1983, Shimizu overcame the delicate existence of the boundary acoustic wave by introducing a film of slow velocity at the interface between the two half-spaces, FIG. 6.
Reference is made to Y. Shimizu and T. Irino, “Stoneley waves propagating along an interface between piezoelectric material and isotropic material,” in Proc. IEEE Ultrasonic Symp., 1983, pp. 373-376, the disclosure of which is herein incorporated by reference. In this reference, Shimizu studies Stoneley waves for an isotropic material like glass and a piezoelectric substrate ZnO. FIG. 6 illustrates Nickel, Gold and Aluminum layers.
The field distribution includes contributions of all three displacements, FIG. 6. For Shimuzu's wave, the polarization is proportionately shear horizontal. A simple illustration of a Shimuzu's boundary acoustic wave propagating in the direction of the X1-axis and decaying in along the X3-axis is illustrated in FIG. 7.
Successful application of SAW filters and duplexers in cellular handsets produced intense competition in the development of high performance with small sizes. In 2006 Murata introduced PBAW based commercial products for the cellular handset market.
Reference is made to H. Kando, D. Yamamoto, M. Mimura, T. Oda, A. Shimizu, K. Shimoda, E. Takata, T. Fuyutsume, R. Kudo and M. Kadota, “RF filter using boundary acoustic wave,” in Proc. IEEE Ultrasonic. Symp., 2006, pp. 188-191, the disclosure of which is herein incorporated by reference.
The physical embodiment of the devices introduced by Murata is illustrated in FIG. 9. The Murata device is conceptually similar to the device by Shimizu. However, the device by Kando et al replaced the layer of Gold at the interface of the half-spaces with a copper IDT. These devices by Murata exhibit strong piezoelectric coupling, lower temperature coefficient of frequency, and eliminate the requirement for a hermetic air cavity package which is a standard requirement for conventional SAW filters. However, these devices require precise manufacturing processes and suffer from larger variations in frequency than do conventional SAW devices.
Reference is now made to K. Yamanouchi and Y. Sato, “Piezoelectric acoustic boundary waves in the structure of multilayer thin films/electrode/piezoelectric substrates,” Journal of Applied Physics, 103, 114105, 2008 and to Y. Sato, D. Malocha and K. Yamanouchi, “Piezoelectric boundary waves in the structure of AlN/SiO2/Electrode/ZnO, AlN/Si substrate,” in Proc. Joint Japan-USA international meeting on acoustic wave devices, 2008, pp. 91-94, the disclosures of which is herein incorporated by reference.
Yamanouchi and Sato recognized the practical advantages of employing a finite SiO2 thickness in combination with an upper half-space of AlN or Al2O3. For example, SAW devices are not constructed on actual half-space dielectrics. Rather, the dielectric needs to be sufficiently thick such that it may be approximated as effectively infinite in thickness. The thickness required for a SiO2 layer to appear effectively infinite is rather extreme and impedes the efficient high volume manufacturing of such devices. By restricting the SiO2 thickness and depositing an AlN or Al2O3 film over the SiO2, Yamanouchi and Sato were able to significantly reduce the thickness of the upper dielectrics to a thickness more attractive for high volume manufacturing.
The physical principle dictating the required thickness of the SiO2, or AlN, or Al2O3 is the penetration depth of the acoustic energy into the layer.
FIG. 10 illustrates the penetration of the boundary wave's acoustic energy in an upper half-space of SiO2, for a device composed of YXLN/Cu-Electrode/SiO2. FIG. 12 Illustrates the penetration of the boundary wave's acoustic energy in an upper half-space of AlN over a SiO2 layer, for a device composed of YXLN/Cu-Electrode/SiO2/AlN.
The decay of the acoustic energy penetrating into the upper half-space is related to the shear wave velocity in the upper half-space. The greater the velocity the more rapid the acoustic energy decays.
Another advantage of using a high velocity material for the upper region of practical PBAW devices is that as the upper dielectric layer decreases in thickness the number of acoustic plate modes is reduced. This effect can be seen in the published work on Y. Wang et al, FIG. 11.
Reference is made to Y. Wang, K. Hashimoto, T. Omori, and M. Yamaguchi, “Analysis of Excitation and Propagation of Piezoelectric Boundary Acoustic Waves in Overlay/Metal Grating/Rotated YX-LiNbO3 Structure,” 2008 Joint Japan-USA International Meeting on Acoustic Wave Devices, pp. 53-58, the disclosure of which is herein incorporated by reference.
However, there remains one substantial impediment to the high volume manufacturing of PBAW type RF filters and duplexers. That is the devices sensitivity to process variation. Since the acoustic energy at the surface of the upper dielectric layer (SiO2, AlN, Al2O3, etc) is negligible, there is no practical process by which the upper surface may be treated in order to adjust the devices center frequency.
There remains a need for PBAW devices that permit a frequency of operation to be adjusted as desired.